A method is provided for determining vibration frequencies of rotating blades mounted on a rotor which rotates relative to and is supported by a rotor support structure. The method includes the steps of: (a) determining possible vibration frequencies of the blade from blade displacements corresponding to the times at which a blade passes a stationary timing probe; (b) converting a vibration signal detectable at the support structure into a vibration frequency spectrum; and (c) identifying the blade vibration frequency by matching a peak in the vibration frequency spectrum with one of the possible blade vibration frequencies.